The invention relates to the design of medication delivery systems with a view to compactness, reliability, weight and production cost.
The invention relates specifically to: A portable medication delivery device comprising a medication cartridge having an outlet and a movable piston, and a housing for holding said cartridge, and a flexible piston rod being operable to engage and displace said piston along an axis of said cartridge, and guiding means for bending said piston rod away from said axis, and actuating means, and driving means for transferring movement from said actuating means to said piston rod, said driving means including a driving wheel for displacing the piston rod, said flexible piston rod comprising regularly spaced first members adapted to mechanically cooperate with corresponding second members on said driving wheel.
The following account of the prior art relates to one of the areas of application of the present invention, medication delivery systems for self-treatment of a disease.
In a medication delivery system for self-treatment of a disease such as diabetes, safety and convenience in the handling of the injection or infusion are of major importance. One very important aspect of this is compactness of the device. Another very important aspect for a user is to ensure that a correct intended dose is delivered.
For the manufacturer of the devices aspects such as production control (testability) and economical solutions are important. A handy, i.e. small volume and lightweight, device that is highly reliable and at the same time economical in production is in demand.
U.S. Pat. No. 5,637,095 discloses a medication infusion pump with a flexible drive plunger comprising a spring tape. In one preferred form of the invention, the spring tape is wrapped or coiled onto a take-up spool within the pump housing. Drive means such as a drive capstan roller and associated pinch roller engage and advance the spring tape under control of the pump drive motor to correspondingly engage and advance the piston to deliver medication to the patient in a programmed manner.
EP-A-0 110 687 discloses a portable fluid infusion device including a flexible piston rod assembly. Attached to the piston is a flexible pusher tape, reversely bent in a U-shape at the upstream or open end of the syringe so that the distal end of the tape is adjacent to and in a position to ride upon the cylindrical body of the syringe as the piston is moved within the body of the syringe. The flexible pusher tape is made of a readily flexible plastics material such as a molded polypropylene. A series of parallel grooves are preformed in the face of the plastics tape along the narrow center portion thereof. The outwardly extending face of the tape is engaged by the helical drive screw at the part of the piston rod, which is deflected away from the axis of the cartridge. The rotation of the screw by a DC motor propels the tape in its U-shaped path to drive the piston.
The problems of the prior art are that the driving means are located relatively far from the piston and require a relatively thick piston rod having a relatively high friction with the housing and the cartridge, thus requiring a relatively high power consuming motor to advance it and possibly introducing inaccuracies in the displacement of the piston, or that the means for advancing the tape-shaped piston rod, the take-up spool storing the unused part of the tape, and the drum or wheel that supports the U-shaped path of the tape are individual self-contained functional units, yielding a relatively complex solution.
The object of the present invention is to provide a medication delivery system that combines compactness with an improved accuracy.
This is achieved according to the invention in that said flexible piston rod exhibits a linear or approximately linear path between said driving wheel and said piston.
In the present context, the term xe2x80x98medication delivery systemxe2x80x99 is taken to mean an injector type device (such as a pen injector or a jet injector) for delivering a discrete dose of a liquid medication (possibly in the form of small drops) or a medication pump for continuous delivery of a liquid medicationxe2x80x94in both cases optionally in combination with relevant electronic monitoring and control and possibly communications units.
In the present context the term xe2x80x98pistonxe2x80x99 is taken to mean a displaceable plate or cylinder that fits tightly against the inner walls of a cartridge. A surface of the piston that faces the inner part of the cartridge and which may be brought into contact with the contents of the cartridge is termed xe2x80x98the inner surface of the pistonxe2x80x99, and the opposite side of the piston is termed xe2x80x98the outer surface of the pistonxe2x80x99. In cooperation with a xe2x80x98piston rodxe2x80x99 that is engaged with xe2x80x98the outer surface of the pistonxe2x80x99, the xe2x80x98pistonxe2x80x99 may be displaced and used to apply pressure to a surface of the contents of the cartridge being in contact with xe2x80x98the inner surface of the pistonxe2x80x99, thus e.g. delivering a dose through the outlet of the cartridge, if the piston is displaced in the direction towards the outlet. In the present context, the term xe2x80x98pistonxe2x80x99 may also apply to a movable wall or membrane that engages with a plunger being an integral part of the piston rod.
In the present context, the term xe2x80x98piston rod being operable to engage and displace said pistonxe2x80x99 is taken to mean that the piston rod may or may not be fixed to the movable wall, but that in both cases it has the ability to displace the piston at least in a direction towards the outlet of the cartridge.
In the present context, the term xe2x80x98driving wheelxe2x80x99 is taken to mean the part of the driving means that cooperates with the activating means (e.g. an electromotor) and the piston rod to transfer the movement of the activating means to a displacement of the piston rod. It may take the form of a gear wheel or drum or any other appropriate form that may be adapted to cooperate with the piston rod. A gear box may be inserted between the activating means and the driving wheel.
An advantage of having a linear path of the piston rod between the driving wheel and the piston is that the risk of introducing errors in the displacement of the piston by the rod is reduced because a more direct drive of the piston is provided (i.e. the sources of mechanical inaccuracies from a remotely located driving wheel due to the curved path, extra friction of the guiding means, etc. are eliminated). Related advantages of this are that an improved dose accuracy may be achieved and that the requirements with respect to the power of the activating means are reduced, leading to a possible reduction in weight and volume of the device.
A further improved dose accuracy may be achieved by combining the present invention with the invention disclosed in our co-pending patent application A medication delivery system with improved dose accuracy incorporated herein by reference.
Other advantages of the invention are that it uses a simple principle for transferring rotational movement of the actuating means to translatory movement of the piston rod, a principle that may be implemented in many different ways depending on the design constraints as regards space and materials. It allows the construction of a relatively compact medication device (utilizing e.g. a 180 degrees curve of the path of the piston rod), using a relatively thin piston rod yielding the benefits of a low weight and a potentially economical solution that is suited for large-scale production.
When said flexible piston rod remains in a fully reversible elastic state during cooperation with said driving wheel and said guiding means, it is ensured that the piston rod is able to adjust to the various curvatures of the driving and guiding means and cooperate therewith, without being irreversibly deformed.
When the surface of said flexible piston rod in a transversal cross-section is adapted to follow the surface of said driving wheel fully or partially, it is ensured that the change of the cross-section of the piston rod during its bending cooperation with the driving and guiding means is accounted for to ensure a seamless cooperation between the rod and the driving and guiding means.
When said guiding means also work as a take-up spool for storing the unused part of the piston rod, it is ensured that an especially compact and economical solution is provided.
When said driving wheel and said guiding means are the same physical unit, it is ensured that an especially simple, compact and economical solution is provided.
When said driving wheel is located after said guiding means in a downstream direction towards said cartridge, it is ensured that the driving means are located as close as possible to the piston to be displaced, thus minimizing the sources of errors and improving dose accuracy.
In the present context, the term xe2x80x98downstreamxe2x80x99 is taken to mean in a direction of the movement of the piston when medication is expelled from the cartridge, said direction also defining a xe2x80x98longitudinal directionxe2x80x99 of the piston rod.
When said piston rod touches the inner walls in an axial direction of said cartridge at one or more points in a transversal cross-section of said piston rod, it is ensured that the ability of the rod to withstand an axial pressure is improved.
In the present context xe2x80x98the inner walls of the cartridgexe2x80x99 are taken to mean the interior surfaces of the walls of the cartridge being in contact with the medication. xe2x80x98A transversal cross-section of the piston rodxe2x80x99 is taken to mean a cross-section of the rod that is perpendicular to the longitudinal direction of the piston rod, i.e. for the part of the rod between the driving wheel and the piston, a direction perpendicular to the direction defined thereby (i.e. perpendicular to the axial direction of the cartridge).
When the longitudinal edges of said piston rod in the cartridge touch the inner walls of said cartridge, it is ensured that the piston rod is guided when displacing the piston in the cartridge, improving the ability to withstand an axial pressure.
In the present context the term xe2x80x98the longitudinal edges of said piston rod in the cartridgexe2x80x99 refers to the edges of the rod in a direction defined by the longitudinal direction of the piston rod.
When said flexible piston rod is tape-shaped, it is ensured that a solution that is convenient from a production point of view and which is well suited for coiling is provided, and that an improved flexibility in the physical design of the medication delivery system is introduced.
In the present context the term xe2x80x98tape-shapedxe2x80x99 in connection with xe2x80x98piston rodxe2x80x99 is taken to mean that the cross-section of the rod perpendicular to its longitudinal direction is xe2x80x98widerxe2x80x99 than its xe2x80x98heightxe2x80x99. It does not have to take the form of a rectangular cross-section but could be grooved or toothed or wave-shaped or convex or concave or something else that might be convenient from a design point of view.
When said flexible piston rod consists of two separate tape-shaped bodies that are joined together at one or more points in a transversal cross-section, it is ensured that a greater axial pressure may be applied to the rod compared to a xe2x80x98single layerxe2x80x99 solution, in other words that thinner materials may be used, resulting in lower weight and reduced costs and thus a greater degree of freedom in the design of the medication delivery device. Further, the piston rod may be subject to a sharper bend than a corresponding one-layer rod. A tape-shaped piston rod that is sufficiently stiff in itself (i.e. without touching the inner walls of the cartridge) may thus be provided.
In a preferred embodiment said two separate tape-shaped bodies are joined together in the central point of a transversal cross-section.
In a preferred embodiment the distance between said two tape-shaped bodies increases with increasing distance from said central point when viewed in a transversal cross-section in a relaxed state.
In the present context the term xe2x80x98in a relaxed statexe2x80x99 refers to a situation in which no external forces (other than gravity) are applied to the rod.
When said two separate tape-shaped bodies are joined together at the edges of a transversal cross-section, it is ensured that an economical and light-weight solution that is well suited for production in larger quantities is provided, and which may be bent around a relatively smaller minimum radius of curvature.
In a preferred embodiment, said edges are joined by applying a coating layer to the surface of said separate tape-shaped bodies. This has the advantage (in addition to joining the two bodies by forming a continuous and flexible hinge between their corresponding longitudinal edges) of yielding a very even surface of the rod and provides the possibility of applying a coating that is especially appropriate for the application in question, e.g. that it minimizes friction, that it improves corrosion resistance, etc.
In a preferred embodiment said two tape-shaped bodies describe an eye-shaped path when viewed in a transversal cross-section in a relaxed state.
In a preferred embodiment said first members on said flexible piston rod comprise protruding members and said corresponding second members on said driving wheel comprise receiving members. In an embodiment thereof, the driving wheel comprises e.g. regularly spaced indentations that interact with corresponding protrusions on the piston rod to displace the piston rod. This has the advantage that the protrusions are xe2x80x98hiddenxe2x80x99 in the driving wheel when the piston rod engages the wheel, possibly reducing the free space needed around the driving wheel (thus minimizing the volume of the construction).
In a preferred embodiment said first members comprise receiving members and said corresponding second members comprise protruding members.
In a preferred embodiment said first members comprise individually isolated indentations and said corresponding second members comprise individually isolated projecting members.
When said first members comprise individually isolated through holes and said corresponding second members comprise individually isolated projecting members, it is ensured that a secure grip between piston rod and driving wheel is provided.
In a preferred embodiment said regularly spaced members are located along a centerline in the longitudinal direction of said flexible piston rod.
When said first members comprise individually isolated cuts located at least along one periphery in the longitudinal direction of said flexible piston rod and said corresponding second members comprise individually isolated projecting members, it is ensured that a secure grip between piston rod and driving wheel is provided. Further, a greater axial stiffness of the rod is achieved.
When the piston rod is at least partially made of a plastics material, it is ensured that a solution combining the benefits of using a plastics material (e.g. corrosion resistance) with those of other materials (e.g. greater mechanical stability, stiffness, etc.) may be provided.
In a preferred embodiment the piston rod is at least partially made of a metallic material.